Control device



y 25, 1944. w. L. LUDWICK 2,354,529

CONTROL DEVICE I Filed June 3, 1942 o v I 1 2 H'METALLF ZSnventor 'WYLLiam L. Ludwick (Ittomcg Patented July 25, 1944 CONTROL DEVICE -William L. Ludwick, East Orange, N. 1., aasignor to Thomas A. Edilo n, Incorporated, West Orange, N. 1., a corporation oi N. 1.

Application June 3, 1942, Serial No. 445,687

Claims. (Cl. 200-122) My invention relates to controlling the thermal characteristics or control devices, and has particular relation'to electrically operated relays oi the thermal type.

It has been very diiiicult in thermal relays oi conventional design to obtain a wide control over the actuating and restoring periods oi the relays-that is, over the operating periods between the times of starting and stopping oi energization oi the relays and the respectively responding opening or closing oi the relay contact points. Moreover, there has been a dimcult problem in obtaining relays having operating periods which are substantially independent oi variations in the ambient temperature. For instance, it has been standard practice to use a housing consisting oi a single glass bulb which is evacuated or filled with a suitable inert gas. A thermal relay with such a housing is iast-acting; when energizing voltage is applied to its heating coil its thermal elements reach very quickly their temperature ior iuli actuation-hereinafter termed the actuating temperature: also the thermal elements cool oi! iairly rapidly once the energizing voltage is removed. When substantially longer operating periods are desired. as .01 the order of several minutes, it has been customary to encase the relay in a block of metal having large heat capacity. This has the eilect oi delaying the rise in temperature oi the thermal elements, but in these arrangements the operating periods ior the relays are in practice limited to a maximum oi about iour or live minutes. This limitation in the operating periods arises irom the fact that-these relay arrangements have a high rate oi heat loss which operates to stabilize the temperature of the thermal elements at a value below that required to actuate the relay. In many control systems however there are desired thermal relays having much longer operating periods as oi a substantial iraction oi an hour or as long as several hours. I have found that such long operating periods can be obtained. As a generality I find that the operating periods of thermal relays can be very accurateiy and flexibly controlled over wide ranges while yet realizing a sensitive and reliable'relay operation, and that this can be accomplished by very simple means. a

It is accordingly an object oi my invention to provide new and improved means ior controlling the thermal characteristics oi control devices and, particularly, the operating periods oi thermal relays.

Another object is to provide improvements in thermal relays which are adapted to permit the operating periods ior the relays to be set at any value over a wide range and to be substantially independent oi variations in ambient temperature.

Another obiect is to provide an improved ther- In the description oi my invention reierence is had to the accompanying drawing oi which:

Figure l is a view oi a thermal relay incorporating my invention, wherein the outer housing oi the relay is in section substantially on the line i-l oi Figure 2;

the relay shown in Figure l; and

' Figures is a view. partly in section and partly broken away. oi the relay proper.

Reierence being had to the accompanying drawing, there will be seen a standard thermostatic unit i which, ior purposes oi illustration,

may comprise a pair oi thermal or bi-metal arms I carrying contact points I and mounted in cantilever iashion within a glass bulb l, the contact points being ior example held in open positions when the unit is in a cool state. The thermal arms are temperature-actuated by a heating coil I which in my invention is preierably mounted externally oi the bulb i, on the outer periphery thereoi. and interposed between insulating wrappings I. This bulb and coil assembly is encased in a highly eihcient heat conservator comprising a closed double-walled glass Jacket or casing I, known otherwise as a Dewar tube, and the assembly and enveloping casing are seated and cemented to a socket or standard tube base I. As so mounted there is iormed a chamber ll between the bulb and the casing l in which is disposed the heating coil 5. The socket is heatinsulated irom the assembly and casing by heat insulation I, which may be rock wool or other suitable insulating material. Lead wires it serve to connect the thermal arms 2 and heating coil i to respectivepairsoi' terminalpins ii and i2 provided on the tube base.

For best heat conservation, the casing I is evacuated to retard any heat escape by conduction or convection and the walls oi the Jacket are silvered as at II to prevent escape oi heat by radiation. The space chamber N between the casing and the bulb l is preferably fllled with a gas having large heat-storage capacity, which may ior example be air. The inner bulb 4 is evacuated or filled with a gas having high heat 56 conductivity. such as hydrogen, the pressure oi filling depending upon the particular thermal characteristics desired as will hereinafter appear. In this arrangement oi my invention the air chamber It acts as a heat storage medium ior O0 actuating the relay. This medium is very highly l igurezisanendviewoithebaseportionoi thermally insulated from the outside ambient by the double-walled casing I and thus the relay has inherently a very long restoring period. Of course the restoring period can be made shorter, as may be desired, by reducing the amount of vacuum in the space between the walls and the casing, by filling the space with gases having particular conductivity characteristics and by providing the jacket without silvering.

The great amount of heat insulation provided the relay by the double-walled casing I enables the actuating period of the relay to be made very long or, if desired, very short. For instance, for a given wattage expenditure in the heating coil the actuating period can be made shorter or longer by selection the conductivity between the air chamber l4 and the thermal arms 2. For a maximum heat conductivity to get a short actuating period, the heating coil may be applied directly to the bulb 4 and the bulb may be fllled to a substantial pressure, say to that or the atmosphere, with a gas having high heat conductivity, such as hydrogen. For a minimum heat conductivity, to get a long actuating period, the heating winding may be interposed between one or more layers of heat insulating material, such as is here shown, and the bulb 4 may be evacuated. Of course, intermediate values for the actuating period can be obtained by introducihg hydrogen in the bulb 4 to more or less pressure and by using more or less thickness in insulating wrapping inside and outside the heating coil.

For good heat conductivity between the air chamber l4 and the thermal arms, the actuating period may also be varied through wide limits with my invention merely by varying the wattage expenditure in the heating coil. For example, when a high wattage is expended in the heating coil the thermal arms will heat up and be actuated very rapidly because of the very small heat loss from the system. On the other hand, when a low wattage is expended in the heating coil, the thermal arms will heat up very slowly but will definitely reach their actuating temperature, so long as the rate of heat input of the coil just exceeds the very slow rate oi. heat loss from the relay container.

As typical values for the ranges of the operating periods of the relay herein described, I flnd that by the expedients above-noted the actuating period may be varied from a few seconds to a substantial fraction of an hour, as of thirty minutes or more; and that the restoration period may be varied from a few minutes to several hours. The longer restoration periods are here obtained reliably, but it will be understood that as the restoration period is made longer better regulation for the energizing voltage of the heating coil is required.

While I preferably place the heating coil outside the inner bulb 4 which contains the thermal arms, and fill the intervening space 14 between the bulb 4 and the outer casing 1 with air to serve as. a heat storage medium, it will be understood that these particular conditions may be varied to obtain special operating characteristics. For example, I may flll the space l4 with one or another fluids having particular heat-conducting ability and heat-storing capacity. Also, I may mount the heating coil inside the bulb 4 to obtain extremely rapid actuating periods. By these and the other expedients hereinabove noted I may also exercise a substantial relative control over the actuating and restoring periods of the relay.

It is seen that by my invention I obtain a wide control over the actuating characteristics of thermal relays and that these characteristics are rendered substantially independent of the ambient temperature. Yet I obtain these improvements very inexpensively at only a small increase in the over-all size of the relay.

While my invention has especial utility in connection with relays oi the thermal type, I intend no unnecessary limitation thereto as my invention has also advantageous application with relay and control systems generally wherein a, high degree of heat insulation of particular operative elements of the systems is required to rule out errors from variations in ambient temperature and to obtain wider temperature-controlling eiiects. Also, it may be understood that many changes and modifications may be made in the particular embodiment of my invention herein shown and described without departing from the scope of my invention, which I endeavor to express according to the following claims.

I claim:

1. A thermal relay comprising a thermally-responsive element, a bulb enclosing said element, a heating coil surrounding said bulb, and a housing for said bulb and heating coil comprising a double-walled casing having the space between the walls thereof substantially evacuated of air, said housing being sealed to said bulb to form therewith a closed chamber for said heating coil, said chamber containing gas.

2. A thermal relay comprising thermally-responsive means; a glass bulb enclosing said responsive means; a heating element applied externally to said bulb; and an outer double-walled casing covering said bulb and heating element, said casing being sealed to said bulb to form an enclosed chamber for said heating element, said chamber containing a fluid adapted to serve as a heat-storage medium for the heating element.

3. A thermal relay comprising a temperatureresponsive element, a bulb enclosing said element and substantially evacuated of air, a heating coil surrounding said bulb, and an enveloping doublewalled glass casing sealed to said bulb to form an enclosed. chamber for said heating coil, said ing heat insulated from said bulb.

4. A thermal relay comprising a temperature-,

responsive element; a bulb enclosing said element and containing a relatively high conductive gas at predetermined pressure; a heating element applied externally to said bulb; and an outer covering sealed to said bulb to form an enclosed chamber ior said heating element, said chamber containing a gas serving as a heat-storing medium for said heating element, and said outer covering being double-walled and having the space between the walls thereof substantially evacuated.

5. A thermal relay comprising a temperatureresponsive element, a bulb enclosing saidelement and containing a fluid of predetermined conductivity; heating means for said element; a double-walled casing enclosing said bulb and having the space between the walls thereof substantially evacuated 0! air, said casing forming a closed chamber about said bulb; and a heatstoring medium in said chamber.

WILLIAM L. LUDWICK. 

